Integrated circuits using superconductors are more suitable for ultrafast processing of digital information. Niobium (Nb) based superconductor integrated circuits using rapid single flux quantum (RSFQ) logic have demonstrated the potential to operate at clock frequencies beyond 700 GHz. However, the Nb-based circuits must operate at temperatures close to 4.2 Kelvin (K), which requires heavy cryocoolers with several kilowatts of input power, which is not acceptable for most electronic applications. Circuits based on high temperature superconductors (HTS) would advance the field. However, reproducible HTS Josephson junctions with sufficiently small variations in device parameters have proved elusive despite 16 years after the discovery of HTS.
The success in HTS Josephson junctions has been limited due to the lack of an adequate thin film technology. Because superconducting integrated circuits utilize a multilayer structure including superconducting, insulating and resistive films, an in situ process in which the HTS is formed directly on the substrate is desirable.
The newly-discovered superconductor material magnesium diboride (MgB2) holds great promise for superconducting electronics, in part, because of its relatively high transition temperature (Tc), at which temperature the respective material becomes superconducting and changes in electrical resistance from a measurable or relatively high value of resistance to a value of zero. This temperature for MgB2, in bulk, can be as high as 39 K. Like the conventional superconductor Nb, MgB2 is a phonon-mediated superconductor with a relatively long coherence length. These properties make the prospect of fabricating reproducible uniform Josephson junctions more favorable for MgB2 than for other high temperature superconductors.
A problem encountered in depositing MgB2 thin films, however, is that sufficient magnesium vapor pressure is necessary to provide for a thermodynamically stable MgB2 phase at elevated temperatures. This problem is complicated when forming boride thin films on substrates containing silicon because of magnesium's propensity to chemically react with silicon at high temperatures. Indeed, previous thermodynamic calculations and investigations indicate that it is not possible to grow magnesium diboride thin films on a silicon substrate (T. He, R. J. Cava and J. M. Rowell, “Reactivity of MgB2 with common substrate and electronic materials”, Appl. Phys. Lett., vol. 80, 2002, 291–293). Magnesium's reactivity with silicon and silicon oxides leads to magnesium-silicon and magnesium-silicon-oxide compounds that contaminate the silicon surface and, in extreme situations, prevents usable magnesium boride films on silicon substrates.
Accordingly, a continuing need exists for the efficient manufacture of boride thin films on silicon. It is further desirable to form high purity, super conducting boride films on silicon by an in situ process to facilitate fabrication of semiconductor electronic components.